A conventional thermoelectric generating device is shown in JP-U-63-162916A. The conventional thermoelectric generating device has a thermoelectric element which utilizes an exhaust gas as a high temperature heat source and an ambient air as a low temperature heat source in order to generate electricity. A bypass pipes branched from an exhaust pipe of an engine through a plurality of branch pipes. Each of the branch pipes has an electric valve which opens and closes the branch pipes. The electric valve closes the bypass pipe respectively when the temperature of the exhaust gas exceeds a predetermined value to control an amount of the exhaust gas flowing in the bypass pipe so that the temperature of the thermoelectric element at the high temperature side is kept under the resisting temperature.
JP-2000-297699A shows another conventional thermoelectric generating device which utilizes an engine coolant as a low temperature heat source.
In the thermoelectric generating device shown in JP-U-63-162916A, an amount of generated current depends on a temperature difference between the exhaust gas and the ambient air when the temperature of the exhaust gas is lower than a predetermined value. Since the temperature difference fluctuates in each situation, there is no technical concept to generate current effectively in a positive way.
In the thermoelectric generating device shown in JP-2000-297699A, since the heat of the exhaust gas is absorbed by the engine coolant, the temperature of the engine coolant may increase over a threshold so that overheating of the engine may be caused. To prevent the overheating, it is necessary to upsize a radiator.